Method for fabricating semiconductor device

ABSTRACT

A method for fabricating semiconductor device includes the steps of: forming a trench in a substrate; forming a first oxide layer in the trench; forming a silicon layer on the first oxide layer; performing an oxidation process to transform the silicon layer into a second oxide layer; and planarizing the second oxide layer and the first oxide layer to form a shallow trench isolation (STI).

BACKGROUND OF THE INVENTION 1. Field of the Invention

The invention relates to a method for fabricating semiconductor device,and more particularly to a method for fabricating a dynamic randomaccess memory (DRAM) device.

2. Description of the Prior Art

As electronic products develop toward the direction of miniaturization,the design of dynamic random access memory (DRAM) units also movestoward the direction of higher integration and higher density. Since thenature of a DRAM unit with buried gate structures has the advantage ofpossessing longer carrier channel length within a semiconductorsubstrate thereby reducing capacitor leakage, it has been gradually usedto replace conventional DRAM unit with planar gate structures.

Typically, a DRAM unit with buried gate structure includes a transistordevice and a charge storage element to receive electrical signals frombit lines and word lines. Nevertheless, current DRAM units with buriedgate structures still pose numerous problems due to limited fabricationcapability. Hence, how to effectively improve the performance andreliability of current DRAM device has become an important task in thisfield.

SUMMARY OF THE INVENTION

According to an embodiment of the present invention, a method forfabricating semiconductor device includes the steps of: forming a trenchin a substrate; forming a first oxide layer in the trench; forming asilicon layer on the first oxide layer; performing an oxidation processto transform the silicon layer into a second oxide layer; andplanarizing the second oxide layer and the first oxide layer to form ashallow trench isolation (STI).

According to an embodiment of the present invention, the silicon layeris formed on the first oxide layer without filling the trenchcompletely.

According to an embodiment of the present invention, the silicon layeris transformed into the second oxide layer to fill the trenchcompletely.

According to an embodiment of the present invention, anatomic layerdeposition (ALD) process is performed to form the first oxide layer.

According to an embodiment of the present invention, the oxidationprocess comprises an in-situ steam generation (ISSG) process.

According to an embodiment of the present invention, a thickness of thesilicon layer is less than a thickness of the first oxide layer.

According to an embodiment of the present invention, a thickness of thesecond oxide layer is greater than a thickness of the silicon layer.

According to an embodiment of the present invention, the silicon layercomprises an amorphous silicon layer.

According to an embodiment of the present invention, the first oxidelayer and the second oxide layer comprise silicon oxide.

These and other objectives of the present invention will no doubt becomeobvious to those of ordinary skill in the art after reading thefollowing detailed description of the preferred embodiment that isillustrated in the various figures and drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 illustrates a top-view of a DRAM device according to anembodiment of the present invention.

FIGS. 2-6 illustrate cross-sectional views for fabricating the DRAMdevice along the sectional line AA′.

DETAILED DESCRIPTION

Referring to FIGS. 1-6, FIGS. 1-6 illustrate a method for fabricating aDRAM device according to an embodiment of the present invention, inwhich FIG. 1 illustrates a top-view of a DRAM device according to anembodiment of the present invention and FIGS. 2-6 illustratecross-sectional views for fabricating the DRAM device along thesectional line AA′. Preferably, the present embodiment pertains tofabricate a memory device, and more particularly a DRAM device 10, inwhich the DRAM device 10 includes at least a transistor device (notshown) and at least a capacitor structure (not shown) that will beserving as a smallest constituent unit within the DRAM array and alsoused to receive electrical signals from bit lines 12 and word lines 14.

As shown in FIG. 1, the DRAM device 10 includes a substrate 16 such as asemiconductor substrate or wafer made of silicon, a shallow trenchisolation (STI) 24 formed in the substrate 16, and a plurality of activeareas (AA) 18 defined on the substrate 16. A memory region 20 and aperiphery region (not shown) are also defined on the substrate 16, inwhich multiple word lines 14 and multiple bit lines 12 are preferablyformed on the memory region 20 while other active devices (not shown)could be formed on the periphery region. For simplicity purpose, onlydevices or elements on the memory region 20 are shown in FIG. 1 whileelements on the periphery region are omitted.

In this embodiment, the active regions 18 are disposed parallel to eachother and extending along a first direction, the word lines 14 ormultiple gates 22 are disposed within the substrate 16 and passingthrough the active regions 18 and STI 24. Preferably, the gates 22 aredisposed extending along a second direction such as Y-direction, inwhich the second direction crosses the first direction at an angle lessthan 90 degrees.

The bit lines 12 on the other hand are disposed on the substrate 16parallel to each other and extending along a third direction such asX-direction while crossing the active regions 18 and STI 24, in whichthe third direction is different from the first direction and orthogonalto the second direction. In other words, the first direction, seconddirection, and third direction are all different from each other whilethe first direction is not orthogonal to both the second direction andthe third direction. Preferably, contact plugs such as bit line contacts(BLC) (not shown) are formed in the active regions 18 adjacent to twosides of the word lines 14 to electrically connect to source/drainregion (not shown) of each transistor element and storage node contacts(not shown) are formed to electrically connect to a capacitor.

The fabrication of STI or isolation structures before the formation ofword lines 14 (or also referred to as buried word lines) is explainedbelow. As shown in FIG. 2, at least a trench such as a trench 26 andanother trench 28 are formed in the substrate 16 on the memory region20, and an atomic layer deposition (ALD) process or chemical vapordeposition (CVD) process is conducted to form a first oxide layer 30 onthe surface of the substrate 16 and into the trenches 26, 28 withoutfilling the trenches 26, 28 completely. In this embodiment, the firstoxide layer 30 is preferably made of silicon oxide and the thickness ofthe first oxide layer 30 is preferably between 50 Angstroms to 90Angstroms or most preferably at 70 Angstroms.

Next, as shown in FIG. 3, a silicon layer 32 is formed on the firstoxide layer 30, in which the silicon layer 32 is disposed on the surfaceof the first oxide layer 30 and also not filling the trenches 26, 28completely. In this embodiment, the silicon layer 32 preferably includesan amorphous silicon layer and the thickness of the silicon layer 32 isslightly less than the thickness of the first oxide layer 30, in whichthe thickness of the silicon layer 32 at this stage is preferablybetween 20 Angstroms to 40 Angstroms or most preferably at 30 Angstroms.

Next, as shown in FIG. 4, an oxidation process 34 is conducted totransform the silicon layer 32 into a second oxide layer 36. In thisembodiment, the oxidation process 34 preferably includes an in-situsteam generation (ISSG) process and the approach of employing ISSGprocess to transform the silicon layer 32 into the second oxide layer 36preferably includes reacting all of the silicon layer 32 with oxygen gasto transform into the second oxide layer 36. In other words, none of thesilicon layer 32 would remain after the ISSG process 34 is completed andthe newly formed second oxide layer 36 would be disposed on the surfaceof the first oxide layer 30 to fill the trenches 26, 28 completely. Inthis embodiment, the second oxide layer 36 and the first oxide layer 30are preferably made of same material such as but not limited to forexample silicon oxide. Next, a planarizing process such as chemicalmechanical polishing (CMP) process and/or etching process is conductedto remove part of the second oxide layer 36 and part of the first oxidelayer 30 to form STI 24 in the trenches 26, 28, in which the top surfaceof the STI 24 is preferably even with the top surface of the substrate16.

Next, fabrication process for forming word lines (or also referred to asburied word lines) could be conducted thereafter. For instance, as shownin FIG. 5, an etching process could be conducted to remove part of theSTI 24 and part of the substrate 16 between or adjacent to the STI 24 toform a first trench 40 and second trench 42, in which the STI 24 isbeneath the bottom of the first trench 40 and the top surface of the STI24 is slightly lower than the bottom surface of the second trench 42.Next, an ALD process, a CVD process, or an ISSG could be conducteddepending on the demand of the process to form a gate dielectric layer44 made of silicon oxide in the first trench 40 and the second trench42.

Next, as shown in FIG. 6, a barrier layer 46 and a conductive layer 48are sequentially formed in the first trench 40 and second trench 42 tofill the trenches 40, 42 completely, and an etching back process isconducted to remove part of the conductive layer 48, part of the barrierlayer 46, and part of the gate dielectric layer 44 so that the topsurface of the remaining conductive layer 48, barrier layer 46, and gatedielectric layer 44 is slightly lower than the top surface of thesubstrate 16. This forms first gate structures 50 in the first trenches40 and second gate structures 52 in the second trenches 42, in which thefirst gate structures 50 and second gate structures 52 essentiallybecome the word lines 14 shown in FIG. 1. Next, a hard mask 54 is formedon each of the first gate structures 50 and second gate structures 52,in which the top surface of the hard masks 54 is even with the topsurface of the substrate 16.

In this embodiment, the barrier layer 46 preferably includes a workfunction metal layer which could be a n-type work function metal layeror p-type work function metal layer depending on the demand of theprocess or product. In this embodiment, n-type work function metal layercould include work function metal layer having a work function rangingbetween 3.9 eV and 4.3 eV such as but not limited to for exampletitanium aluminide (TiAl), zirconium aluminide (ZrAl), tungstenaluminide (WAl), tantalum aluminide (TaAl), hafnium aluminide (HfAl), ortitanium aluminum carbide (TiAlC), but not limited thereto. P-type workfunction metal layer on the other hand could include work function metallayer having a work function ranging between 4.8 eV and 5.2 eV such asbut not limited to for example titanium nitride (TiN), tantalum nitride(TaN), or tantalum carbide (TaC), but not limited thereto. Theconductive layer 48 could be made of low resistance material includingbut not limited to for example Cu, Al, W, TiAl, CoWP, or combinationthereof and the hard masks 54 are preferably made of dielectric materialsuch as silicon nitride.

Next, an ion implantation process could be conducted depending on thedemand of the process to form a doped region (not shown) such as lightlydoped drain or source/drain region in the substrate 16 adjacent to twosides of the first gate structure 50 or second gate structure 52. Next,a contact plug process could be conducted to form word line contactsadjacent to two sides of the second gate structures 52 electricallyconnecting the source/drain region and bit lines formed thereafter andstorage node contacts electrically connecting the source/drain regionand capacitors fabricated in the later process.

Overall, the present invention first form a trench in the substrate anda first oxide layer in the trench before the buried word line (BWL) of aDRAM device is fabricated, forms a silicon layer or more specifically anamorphous silicon layer on the surface of the first oxide layer, andthen conducts an ISSG process to transform all of the amorphous siliconlayer into a second oxide layer, and then planarize part of the secondoxide layer and part of the first oxide layer to form a STI. Accordingto a preferred embodiment of the present invention, this approach notonly reduces the chance of creating seams in the STI made of siliconoxide, but also prevents over consumption of silicon substrate when theISSG process is conducted.

Those skilled in the art will readily observe that numerousmodifications and alterations of the device and method may be made whileretaining the teachings of the invention. Accordingly, the abovedisclosure should be construed as limited only by the metes and boundsof the appended claims.

What is claimed is:
 1. A method for fabricating semiconductor device,comprising: forming a trench in a substrate; forming a first oxide layerin the trench, wherein the first oxide layer is a single-layeredstructure made of silicon oxide; forming a silicon layer on and directlycontacting the first oxide layer without filling the trench completely,wherein a thickness of the silicon layer is less than a thickness of thefirst oxide layer; performing an oxidation process to transform thesilicon layer into a second oxide layer for filling the trenchcompletely, wherein a maximum width of the second oxide layer in thetrench is greater than a maximum width of the silicon layer in thetrench; and planarizing the second oxide layer and the first oxide layerto form a shallow trench isolation (STI).
 2. The method of claim 1,further comprising forming the silicon layer on the first oxide layerwithout filling the trench completely.
 3. The method of claim 1, furthercomprising transforming the silicon layer into the second oxide layer tofill the trench completely.
 4. The method of claim 1, further comprisingperforming an atomic layer deposition (ALD) process to form the firstoxide layer.
 5. The method of claim 1, wherein the oxidation processcomprises an in-situ steam generation (ISSG) process.
 6. The method ofclaim 1, wherein a thickness of the silicon layer is less than athickness of the first oxide layer.
 7. The method of claim 1, wherein athickness of the second oxide layer is greater than a thickness of thesilicon layer.
 8. The method of claim 1, wherein the silicon layercomprises an amorphous silicon layer.
 9. The method of claim 1, whereinthe first oxide layer and the second oxide layer comprise silicon oxide.